Still, if you have a list of Pathfinder books you're considering adding to PCGen, ISG would be my nomination (purely because it'll help ease the bookkeeping for the Council of Thieves campaign I'm in at present...)

Some stuff from this book is easy to set up, such as feats, spells, traits, and equipment. However, the three new prestige classes bring about some a game mechanism—i.e. the character keeps acquiring class features from one of his bas classes when leveling up in the prestige class—that is probably not supported by PCGen code currently.